A. D, MICHAEL ON THE ACARINA. 225 



M. Robin watched these creatures, and satisfied himself that copu- 

 lation did not take place with the adult female, which possesses the 

 conspicuous external vulva with its horse-shoe shaped sternite, but al- 

 ways apparently with the nymph, and it is, I believe, to his investiga- 

 tions that we owe the explanation which appears likely to prove the 

 true one, namely, that although there is only one form of adult male, 

 there are two forms of what must be considered adult females, 

 differing in appearance, and one succeeding the other ; and that 

 fecundation takes place in the first stage only and not in the second, 

 while the second form alone lays eggs. The first form, which bears 

 the closest resemblance to the nymph, is called by Robin " femelle 

 accouplee" I speak of it in English as the " nubile female." Coition 

 is effected with this nubile female at the anal region, but it was a 

 doubtful question whether there was a pseudo-vulval opening close 

 to the anus or whether fecundation took place by the anus itself. 

 M. Megnin, who has carefully investigated the subject, is of opinion 

 that the latter is the true explanation, the anus being a cloaca, as in 

 birds, &c, and that the vulva is used solely lor the deposition of 

 eggs. The second form is called by M. Robin "femelle fecondee? 

 a term which I prefer to translate as " reproductive female," as I 

 am not aware that there is any proof that the change does not take 

 place in the absence of actual fecundation. It will be seen that this 

 form in P. glandarinus and others is manifestly unsuited for copu- 

 lation by the anus, that part being protected by strong spikes, which 

 give the creature a singular appearance. 



It will naturally be asked what proof exists that the coition is 

 not with the nymph after all, in spite of the apparent improbability. 

 This I understand to be answered by Messrs. Robin and Megnin as 

 follows : — Firstly, that it being improbable, the onus of proof 

 would rather rest upon those who assert that it does exist. Secondly, 

 that although the nubile female is very like the nymph, yet a care- 

 ful examination will show that it is not identical, but that certain 

 small differences often exist, such as the larger size of the nubile 

 female, and the presence in certain species of two small, cylindrical 

 appendages at the anal end of the body of the nubile female, which 

 do not exist in the nymph, and which are sufficient to show the dis- 

 tinction. Thirdly, that as far as has been ascertained, the female 

 nymph casts its skin as often as the male, without counting the final 

 change to the reprodu* tive female, and that, on this final change, no 

 apparent dissolution and re-formation takes place and that it is only 



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